Simple definition
A MU-plugin (Must-Use Plugin) is a WordPress plugin that loads automatically without requiring activation from the Plugins screen.
Technical definition
PHP files placed directly in wp-content/mu-plugins are automatically loaded by WordPress before standard plugins. They cannot be disabled from the normal Plugins interface. Subdirectories are not loaded automatically unless a loader file in the root explicitly includes them.
How it works / role
Its role is to host functionality that must stay active regardless of normal administrator plugin actions, such as hosting bootstrap code, security controls, multisite configuration, or essential business logic.
What is it used for?
Ensure that critical code is loaded on every WordPress request without depending on manual plugin activation.
Practical example
A hosting provider installs a MU-plugin that enforces caching and security settings. Even if an administrator disables all standard plugins, the MU-plugin continues to load.
Common issues
- MU-plugin overlooked because it is not managed like a standard plugin
- PHP error in a MU-plugin affects the whole site
- Code placed only in a subdirectory without a root loader file
- Conflict with a standard plugin that runs later
Key takeaway: When troubleshooting WordPress, always inspect wp-content/mu-plugins: a MU-plugin can affect the site even when all standard plugins are disabled.